1. Field of the Invention
This invention concerns the manufacture of glass preforms for drawing optical fibers.
2. Discussion of the Known Art
To reduce costs associated with the manufacture of optical fiber preforms, as few steps as possible should be performed when carrying out the manufacturing process. Yet, at present, a large number of processing steps are actually required when fabricating such preforms. For example, in order to produce AllWave® optical fiber available from OFS Fitel, a glass preform is initially fabricated by:
1. Making an original core body using, e.g., a known vapor axial deposition (VAD) process;
2. Stretching the VAD core body into an intermediate core rod for analysis;
3. Performing an index profile measurement, and stretching the core rod further to a determined size; and
4. Jacketing the core rod with an overcladding glass tube or cylinder of proper size.
See U.S. Pat. No. 6,131,415 (Oct. 17, 2000). See also, U.S. Pat. Application Pub. No. 2006/0216527 (Sep. 28, 2006) which discloses an optical fiber preform that is assembled by inserting a number of core rod segments inside an innermost one of two or more concentric overclad tubes, and U.S. Pat. No. 6,460,378 (Oct. 8, 2002) disclosing a multiple overclad optical fiber preform. All relevant portions of these patent references are incorporated herein by reference.
When drawn from a preform, the above mentioned AllWave fiber has an outside (cladding) diameter of 125 microns, with a typical inner core diameter of 8.3 microns. A key feature of the AllWave fiber is that its optical loss at 1385 nm wavelength does not exceed 0.31 dB/km. This low loss is attributable mainly to a reduced level of hydroxyl (OH) impurities in the fiber glass, especially within the fiber's core or near the core region.
The original VAD core body obtained in step 1 above is typically made with an outer cladding layer that surrounds the inner core. The ratio of the outside diameter of the cladding layer to the diameter of the core is called the D/d ratio for the core body, wherein D is the diameter of the outer cladding and d is the diameter of the inner core. A typical VAD core body has a D/d ratio in a range of from 3 to 6. But core bodies with D/d ratios of 4 or less (referred to as “low D/d ratio” core bodies) are not uncommon and are actually preferred, since they achieve a greater fiber yield than core bodies of the same size having higher D/d ratios. That is, as the D/d ratio increases for a core body of a given size by reducing the core diameter (d), the yield of useful fiber that can originate from the core body is also reduced and the overall cost of producing a given quantity of fiber increases significantly.
Steps 2 and 3 above involve many glass working procedures and are very time consuming. Also, because the original VAD core body may not always have a uniform cross section over its entire length, it is difficult to achieve a uniform diameter for the intermediate core rod in step 2. Unwanted scrap therefore results.
In step 4, the forming and the preparation of the overcladding glass cylinder also involves many steps and high costs. For example, a typical overcladding cylinder is made as follows:
A. Provide a target rod or mandrel suitable for soot deposition. Such a mandrel is typically formed of alumina.
B. Deposit silica glass soot on the mandrel until a determined weight of the deposited soot is attained. The deposited soot object may weigh up to several hundred pounds.
C. Upon completion of the soot deposition, the alumina mandrel is removed so that a central bore remains in the soot object.
D. The soot object is placed in a sintering furnace for dehydration with the aid of a special fixture, and the object is then consolidated at an elevated temperature near 1500 degrees C. The consolidated object is typically referred to as a glass cylinder.
E. Because the wall of the central bore in the cylinder is typically rough, mechanical grinding and honing is needed to ensure that the wall surface is sufficiently smooth.
F. Because multiple torches are typically used for the soot deposition process in step B, the outside diameter (OD) of the cylinder is usually not uniform over the length of the cylinder. Mechanical grinding of the cylinder's outside circumference then becomes necessary to reduce such variations in the cylinder OD.
G. The cylinder is stretched down to a determined size for overjacketing of the core rod obtained in step 3 above.
Accordingly, many costly steps are involved in producing the overjacketing cylinders, with much associated scrap. One way to reduce these costs would be to use a direct soot overcladding process on the intermediate core rod, and thus eliminate a number of the processing and machining steps associated with the glass cylinder.
For a low D/d ratio core rod it is important in step 4 above to ensure that the interface between the outer cladding layer of the rod and the overjacketing cylinder, remains clean and free of contamination. The quality of this interface is critical with respect to the performance of optical fiber drawn from the finished preform. Direct soot over-cladding deposition on a low D/d ratio core rod will typically generate a so-called “wet” glass layer on the surface of the rod due to the presence of hydrogen and oxygen in the torch flames used to deposit the soot cladding. For a low D/d ratio core rod, the wet glass layer acts to prevent drawing of so-called low water or zero water peak fiber (e.g., the mentioned AllWave fiber) from the preform, due to an overlap of the wet glass layer with regions of the fiber cladding that are near the core and in which light is also transmitted.
In summary, two characteristics of the current procedures for making glass preforms for optical fibers contribute to high production costs; namely, (1) the relatively large number of costly and time-consuming process steps, and (2) the fabrication of the glass cylinder for overjacketing the core rod. Because of the low D/d ratios of the core rods, costs associated with the cylinder in steps 3 and 4 above cannot be eliminated simply by substituting a direct soot overcladding deposition process on the intermediate core rod obtained in step 2. Such a process would introduce the mentioned wet glass layer into the rod and impair fiber performance.